[3] Archaeopteryx lived in the Late Jurassic around 150 million years ago, in what is now southern Germany, during a time when Europe was an archipelago of islands in a shallow warm tropical sea, much closer to the equator than it is now.

However, in recent years, the discovery of several small, feathered dinosaurs has created a mystery for palaeontologists, raising questions about which animals are the ancestors of modern birds and which are their relatives.

[11] The first skeleton, known as the London Specimen (BMNH 37001),[17] was unearthed in 1861 near Langenaltheim, Germany, and perhaps given to local physician Karl Häberlein in return for medical services.

[14] Missing most of its head and neck, it was described in 1863 by Richard Owen as Archaeopteryx macrura, allowing for the possibility it did not belong to the same species as the feather.

[31][32] After a lengthy period in a closed private collection, it was moved to the Museum of Evolution at Knuthenborg Safaripark (Denmark) in 2022, where it has since been on display and also been made available for researchers.

[35] Long in a private collection in Switzerland, the Thermopolis Specimen (WDC CSG 100) was discovered in Bavaria and described in 2005 by Mayr, Pohl, and Peters.

[38][39] Palaeontologists of the Ludwig Maximilian University of Munich studied the specimen, which revealed previously unknown features of the plumage, such as feathers on both the upper and lower legs and metatarsus, and the only preserved tail tip.

[46] Beginning in 1985, an amateur group including astronomer Fred Hoyle and physicist Lee Spetner, published a series of papers claiming that the feathers on the Berlin and London specimens of Archaeopteryx were forged.

[51] Most of the specimens of Archaeopteryx that have been discovered come from the Solnhofen limestone in Bavaria, southern Germany, which is a Lagerstätte, a rare and remarkable geological formation known for its superbly detailed fossils laid down during the early Tithonian stage of the Jurassic period,[55] approximately 150.8–148.5 million years ago.

[68] This would mean that the skin already was softened and loose, which is bolstered by the fact that in some specimens the flight feathers were starting to detach at the point of embedding in the sediment.

[69] Using scanning electron microscopy technology and energy-dispersive X-ray analysis, the team was able to detect the structure of melanosomes in the isolated feather specimen described in 1861.

[70] In a 2013 study published in the Journal of Analytical Atomic Spectrometry, new analyses of Archaeopteryx's feathers revealed that the animal may have had complex light- and dark-coloured plumage, with heavier pigmentation in the distal tips and outer vanes.

[80] Below is a cladogram published in 2013 by Godefroit et al.[3] Aurornis Anchiornis Archaeopteryx Xiaotingia Jeholornis Rahonavis Balaur Avebrevicauda (includes modern birds) It has been argued that all the specimens belong to the same species, A.

The discovery of the closely related Xiaotingia in 2011 led to new phylogenetic analyses that suggested that Archaeopteryx is a deinonychosaur rather than an avialan, and therefore, not a "bird" under most common uses of that term.

[2] A more thorough analysis was published soon after to test this hypothesis, and failed to arrive at the same result; it found Archaeopteryx in its traditional position at the base of Avialae, while Xiaotingia was recovered as a basal dromaeosaurid or troodontid.

The authors of the follow-up study noted that uncertainties still exist, and that it may not be possible to state confidently whether or not Archaeopteryx is a member of Avialae or not, barring new and better specimens of relevant species.

[89] Phylogenetic studies conducted by Senter, et al. (2012) and Turner, Makovicky, and Norell (2012) also found Archaeopteryx to be more closely related to living birds than to dromaeosaurids and troodontids.

[90][91] On the other hand, Godefroit et al. (2013) recovered Archaeopteryx as more closely related to dromaeosaurids and troodontids in the analysis included in their description of Eosinopteryx brevipenna.

The lack of a bony breastbone suggests that Archaeopteryx was not a very strong flier, but flight muscles might have attached to the thick, boomerang-shaped wishbone, the platelike coracoids, or perhaps, to a cartilaginous sternum.

The presence of "hind wings", asymmetrical flight feathers stemming from the legs similar to those seen in dromaeosaurids such as Microraptor, also would have added to the aerial mobility of Archaeopteryx.

[100] In 2010, Robert L. Nudds and Gareth J. Dyke in the journal Science published a paper in which they analysed the rachises of the primary feathers of Confuciusornis and Archaeopteryx.

[108] Archaeopteryx had a cerebrum-to-brain-volume ratio 78% of the way to modern birds from the condition of non-coelurosaurian dinosaurs such as Carcharodontosaurus or Allosaurus, which had a crocodile-like anatomy of the brain and inner ear.

Some scientists see it as a semi-arboreal climbing animal, following the idea that birds evolved from tree-dwelling gliders (the "trees down" hypothesis for the evolution of flight proposed by O. C. Marsh).

[21] In March 2018, scientists reported that Archaeopteryx was likely capable of a flight stroke cycle morphologically closer to the grabbing motion of maniraptorans and distinct from that of modern birds.

[112][113] This study on Archaeopteryx's bone histology identified biomechanical and physiological adaptations exhibited by modern volant birds that perform intermittent flapping, such as pheasants and other burst flyers.

Some researchers suggested that the feather sheaths of Archaeopteryx shows a center-out, flight related moulting strategy like modern birds.

[114] Kiat and colleagues reinterpreted this purported moulting evidence to be problematic and equivocal at best, and considered that these structures more likely represents the calami traces of the fully grown feathers,[115] though the original authors still remained by their conclusion.

[118] The richness and diversity of the Solnhofen limestones in which all specimens of Archaeopteryx have been found have shed light on an ancient Jurassic Bavaria strikingly different from the present day.

The latitude was similar to Florida, though the climate was likely to have been drier, as evidenced by fossils of plants with adaptations for arid conditions and a lack of terrestrial sediments characteristic of rivers.

[120] The pterosaurs included species such as Rhamphorhynchus belonging to the Rhamphorhynchidae, the group which dominated the ecological niche currently occupied by seabirds, and which became extinct at the end of the Jurassic.